The development of the laser interferometer has provided an important advancement which has increased our ability to measure distances easily and precisely. The laser is no longer a laboratory curiosity. Rather, the laser interferometer is now a practical measuring tool for use in industrial environments. Currently, there are several various types of laser interferometer systems available, each utilizing some distinct properties of light and optics. However, regardless of which interferometric technique is used, when the measurement is to be conducted in an air environment, as is most practical in many applications, all such devices are limited by inaccuracies resulting from changes in the refractive index of the air through which the laser beams travel. This is due to the fact that laser wavelength in air is a function of the refractive index of the air, which in turn is dependent on pressure, humidity, temperature, gas content of the air, and the like. Variations in atmospheric conditions will change the value of the refractive index, thus affecting the measurement.
Because the measurements made by interferometric methods are substantially affected by the air refractive index, it is known in the art, when accurate measurements through air are required, to measure the air refractive index and to account therefor in analyzing the data obtained through interferometric measurements. Conventionally, the refractive index of air is determined using sensors to monitor the pressure, temperature and humidity of the ambient air, and the air refractive index is calculated based upon the results of such measurements. This calculation is commonly based upon an equation which was determined by one Edlen using data obtained empirically.
The above described conventional method for determining the air refractive index is quite adequate and useful for applications which do not approach the limits of accuracy of modern laser interferometers. However, cumulative errors in sensor data, and the fact that the empirically determined calculation is not exact, contribute to inaccuracies which may be quite significant in relation to precise measurements. Furthermore, this method does not take into account at all the fact that the ratio of the various species constituents of the air is not constant. Different gas species may have markedly different indexes of refraction. For example, carbon dioxide has a very high index of refraction, and so a small variation in carbon dioxide content in the air can considerably alter the wavelength readings of laser beams traveling through that air.
Yet another problem encountered using conventional methods for measuring the index of refraction of air is that there may be a significant time delay between the taking of readings and the calculated result. Therefore, the correction factor calculated by such means will lag behind the measurement data.
Clearly, a means for quickly and accurately determining the precise instantaneous index of refraction of the ambient air would be desirable. The present inventor has taught a means for using a laser interferometer to directly measure the index of refraction of air in a copending application Ser. No. 07/647,735. U.S. Pat. No. 4,847,512 issued to Seta teaches an application of a laser interferometer for indirectly measuring humidity (one of the factors affecting the index of refraction of air) by the method of using the interferometer to directly measure the index of refraction of the air. Of the means known to the inventor by which a laser interferometer might be utilized to measure the index of refraction of air, or a component thereof, while each is quite well suited to its intended purpose, none is readily adaptable to be used quickly and easily in conjunction with an interferometer, the primary purpose of which interferometer is to make some direct measurement not specifically related to the index of refraction measurement.
All of the prior art means for measuring the index of refraction of air known to the inventor have been less than sufficiently precise, or else they have been awkward and time consuming to use in practical field applications. To the inventor's knowledge, no prior art method for measuring the index of refraction of air has been readily adaptable for use with a conventional measurement interferometer without disrupting the normal usages of the interferometer or requiring that the interferometer be reconfigured to accomplish the measurement of the index of refraction.